The present invention relates generally to semiconductor manufacturing processes, and more specifically to monitoring and determining copper contamination during the manufacture of integrated circuits on semiconductor wafers.
Semiconductor devices are very vulnerable to several different types of contaminants that can affect the physical and electrical properties of the device by lowering the processing yield, changing device performance, and decreasing the device reliability. Yields decrease when contaminants alter the inherent properties of the various material layers and the cleanliness of surfaces. Device parametric values, such as threshold voltage or capacitance, are also changed by such contaminants. Since certain metallic contaminants are mobile, they may not be detected under typical device testing immediately after fabrication, but gradually diffuse inside the device and settle in electrically sensitive areas, causing failures long after the test process has been completed and the device is in use.
Contaminants that are inadvertently added during the processing of a device can be broadly classified into particles and chemical/metallic contaminants. Many such contaminants are present in the various chemicals and fabrication equipment commonly used in semiconductor processing. Due to the ever decreasing feature sizes and thinner deposition layers, devices continue to suffer increasing vulnerability to such contaminants. Because the desired effects of a semiconductor device are achieved using very small quantities of dopants, small quantities of electrically active contaminants can alter the electrical characteristics, changing the device performance and its reliability parameters. A major source of such contaminants are ionic contaminants, that is, atoms and minerals that exist in the semiconductor in ionic form. These metallic ions are highly mobile in the semiconductor material and can therefore cause the device performance characteristics to change during operation, although the device had previously passed electrical testing immediately following fabrication. All fabrication process operators strive to reduce contaminants and to detect those that find their way into the process stream.
It is known that copper contamination of an integrated circuit can significantly change the device functional characteristics. For example, unexpected copper contaminants can cause the capacitance of a contaminated wafer site to differ significantly from the desired design capacitance. Also, like all semiconductor crystal impurities, copper contamination introduces one or more discrete energy levels into the silicon and/or dielectric band gaps between the conduction band and the valence band. These defects form generation/recombination centers (also referred to as traps) that affect the device operation, acting as recombination centers when there are excess carriers in the semiconductor, and as generation centers when the carrier density is below an equilibrium value (e.g., in the reverse-biased space-charge region of a pn junction).
The present trend in the integrated circuit fabrication industry is moving toward more frequent use of copper damascene interconnect processes, and the decreased use of aluminum interconnects. As a result, there are increased opportunities for copper contamination during the fabrication process. This is especially problematic because copper, among all transition metal impurities, exhibits the highest diffusivity and solubility in silicon. Even trace copper contamination can significantly degrade circuit performance and fabrication yields. As a result, during the fabrication process, barrier layers are employed to prevent diffusion of copper from a copper interconnect into an active device area. But trace copper contaminants can also find their way into the active areas through the use of common metrology and cosmetic inspection tools and fabrication facilities (e.g., diffusion or rapid thermal anneal furnaces) for both copper and non-copper interconnected integrated circuits. During testing, some of these tools require physical contact to a chip""s metal layer, resulting in adhesion of residual metal particles on the tool after the test has been completed. For example, electrical probe tips frequently show signs of copper contamination after use on a copper-interconnected wafer. Cross-contamination of copper occurs later when the tool is used on a device active area during the fabrication process. In process furnaces, copper is readily volatized even at high temperatures, due to the gettering effect of the furnace ambients, contaminating active surfaces of wafers in the thermal processing tube and adhering to the thermal processing tube walls, possibly contaminating future wafers processed through the furnace. A single copper-contaminated chamber or tool may affect front and/or back end processing of hundreds of subsequent wafers. One potential solution, the purchase of two sets of tools, one for copper-interconnected integrated circuits and the other for non-copper interconnected circuits, is generally considered cost prohibitive.
One prior art technique for measuring copper and other contaminant densities is x-ray fluorescence (XRF). It is known that incident x-rays are absorbed, emitted, reflected or transmitted from a solid. In the x-ray fluorescence process, incident x-rays on the sample are absorbed by ejecting electrons from the atomic K-shell. Then electrons from higher levels, for example the L-shell, drop into the K-shell vacancies and the liberated energy is emitted as characteristic secondary x-rays. The total reflection XRF (TXRF) technique, illustrated in FIG. 1, is one in which x-rays strike the sample at a very shallow angle, penetrating only a small distance into the sample. An x-ray source 12 produces x-rays that are targeted, via a path 14a, to a monochromator 16, which filters out all but a specified x-ray wavelength and focuses the resulting x-ray signal, via a path 14b, to a specified point 18 on a wafer 20. The reflected x-rays are illustrated by a path 14c. Standing waves are formed above the wafer 20 and detected in a detector 22, which is located within about one mm of the wafer surface. Due to the near total reflection of the incident x-ray, the wafer 20 contributes very little to the spectrum in the standing wave. The x-ray energy in the standing wave identifies the impurity type, and the energy intensity provides a measure of the impurity density. Typical analysis areas are one cm2 although advanced instruments can analyze areas as small as 10xe2x88x926 to 10xe2x88x925 cm2.
To determine the presence of contaminants, a typical TXRF tool requires approximately 1000 seconds to monitor a four mm2 wafer region. Because a single four mm2 area on an eight inch wafer is a relatively insignificant amount of surface area from which to determine whether or not contamination exists across the entire wafer, a number of such regions must be tested by the TXRF tool. Typically, when looking for copper contaminants, a minimum of three to nine sites are checked. If five sites are checked, the process consumes a total of 5000 seconds or approximately 80 minutes to accurately determine whether or not contamination exists at the five sites. Because this detection time is relatively long compared with other steps in the integrated circuit fabrication process, it is not feasible to test more than about nine sites on a wafer and further not feasible to test every wafer for contamination in an in-line process. Also, the information produced by the TXRF analysis requires several days of evaluation to finally yield detailed copper contamination results. Because the process checks only a few wafer regions, the process does not yield accurate data as to the distribution of the copper contamination across the wafer. In fact, the TXRF tool can miss contaminants because it checks only a limited number of discontinuous wafer regions. For example, if contamination exists on an edge of the wafer that was not chosen to be evaluated, likely the wafer would be passed as uncontaminated, when in reality there is significant contamination that can be passed along to other wafers during the fabrication process. Ideally, quantitative data as to the distribution of contaminants across the wafer surface is desired.
To overcome certain drawbacks of the TXRF process, statistical sampling and analysis can be utilized to provide insight into the distribution of the copper contaminants across the wafer surface. For instance, a wafer can be pulled from the fabrication process at given intervals and checked for copper contaminants. After creating a historical contaminant data base, statistical analysis will determine the likelihood and extent of copper contamination in the fabrication process. But in some fabrication facilities, wafer analysis is not conducted until a tool is believed to be contaminated. Wafer analysis at this point cannot detect past contamination, during which significant cross-contamination of fabrication tools can occur, with the attendant reduction in yield and reliability. It should also be noted that when a wafer is removed from the line for a contamination analysis, other wafers continue to be processed through a potentially contaminated tool. Thus numerous additional wafers may be contaminated while the test wafer is undergoing analysis. For example, if a metal deposition and the subsequent chemical/mechanical polishing process takes six minutes per wafer, while the contamination analysis requires 80 minutes per wafer, as many as ten wafers will be subjected to a contaminated metal deposition system before the contamination analysis on the test wafer is completed.
The TXRF technique discussed above identifies contaminants in a range of about 109 to 1010 cmxe2x88x922. This sensitivity can be further improved through hydrofluoric acid condensation, also referred to as vapor phase decomposition TXRF. In this technique, the wafer includes either a native or thermally grown oxide, which produces water as byproduct upon exposure to hydrofluoric acid vapor. The hydrofluoric acid etches the oxide, including the impurities therein, and the resulting water droplets or moisture film also contains the impurity elements. The impurities are collected in situ by scanning the surface with a second water droplet. The residue is allowed to dry and is measured by the TXRF process. Inherently, the process assumes that the water droplet carries all the surface contaminants with it, and it enhances the sensitivity to about 108 cmxe2x88x922, for iron, for example.
One device parameter that is specifically affected by metal contaminants is the minority carrier diffusion length. Iron and copper contaminants are very detrimental as they have a significant affect on this parameter. Other metals including, chromium, molybdenum, nickel and cobalt, are present in smaller quantities, and therefore have a lesser impact on the diffusion length. In addition to affecting the minority carrier diffusion length, metallic contamination at the semiconductor-oxide interface (for instance of a MOSFET device) degrades the oxide integrity. Iron contamination increases the oxide defect density significantly. Metals also degrade device performance if located at high stress points and in space charge regions created at the interface of opposite-conductivity materials. In addition to the heavy metal contaminants affect on diffusion length, oxygen precipitated silicon crystalline defects and oxidation induced defects, especially stacking faults, also contribute to minority carrier diffusion length degradation.
The diffusion length of a p-type semiconductor wafer is measured to determine the copper contaminant concentration. In a p-type material is influenced by various contaminants and other physical characteristics of the semiconductor. For example, the diffusion length could be affected by iron and copper, and many other contaminants to a lesser extent. To isolate the diffusion length associated with the copper, it is first necessary to separate the iron and copper effects from the others. This is accomplished by optically activating (or alternatively, thermally activating) the wafer to change only the copper and iron states. The iron is then allowed to repair to its original state, thus isolating the effects of the copper contamination on the diffusion length. By comparing the initial diffusion length prior to activation and the diffusion length after the iron has repaired to its pre-activation state, the copper-induced diffusion length effects can be isolated and from this diffusion length value, the copper contaminant concentration can be determined.